1. Statement of the Technical Field
The invention concerns timing synchronization of non-GPS equipment, and more particularly synchronizing the time of day clock used on non-GPS type equipment based on a time of day signal output from a GPS receiver.
2. Description of the Related Art
Global positioning systems are typically comprised of a plurality of satellites and a global positioning system (GPS) receiver. GPS satellites have onboard atomic clocks. The satellites also transmit a radio signal that includes the precise time according to the onboard atomic clock. The atomic clocks on the satellites are set to “GPS time”. By definition, GPS time is the number of seconds since 00:00:00 UTC, Jan. 6, 1980. GPS receivers apply to this received signal a clock-correction offset (which is periodically transmitted along with the other data) in order to accurately determine the correct UTC time. The GPS receiver can also automatically adjust for a local time zone. Consequently, a GPS receiver can serve as a highly accurate time clock. In fact, one of the most common applications of the GPS system is for use as a time of day (TOD) reference clock.
Some GPS receivers can include a digital output that can include such TOD information. For example, the GPS receiver can generate an output in a format defined by the National Marine Electronics Association (NMEA). One such format is defined as NMEA 0183 compliant data. Such data is typically output on an RS-232 interface as an asynchronous serial data transmission at a baud rate of 4800.
NMEA 0183 compliant data includes several approved types of data sentences, including one which is referred to as GPRMC. The GPRMC sentence includes, among other items of information, TOD data determined by the GPS receiver. The TOD data is generally expressed as UTC time. Notably, this TOD data can be useful for synchronizing non-GPS equipment.
While GPS based time clock systems are quite common, there remain some problems associated with the use of TOD data provided by a GPS receiver. This is particularly true where a device external to a GPS receiver needs a very precise and accurate source of time. One such problem relates to variable delays and timing errors introduced by the GPS receiver in connection with the processing and transmission of TOD data from the GPS receiver to the external device. Consequently, a systematic method is needed to correct or at least minimize these errors.